Rear conversion lens

Abstract

A rear conversion lens for a single lens reflex camera, constructed with a front lens group including a positive lens with its surface having a greater curvature facing to the image side, a negative biconcave lens, and a positive lens with its surface having a greater curvature facing to the object side, and a rear lens group including a negative lens, the rear conversion lens being provided between an objective lens and a camera body to extend a composite focal length, and satisfying certain conditions.

I claim: 
     
       1. A rear conversion lens for a single lens reflex camera capable of maintaining excellent image forming performance, even if it is mounted on an objective lens having a relatively short back focus, a large aperture ratio, and an exit pupil which is at a relatively distant position from the image plane, comprising: a front lens unit including, in the sequence from the object side, a positive lens, a negative biconcave lens and a positive lens; and   a rear lens unit including a negative lens with its surface of sharper curvature facing the object side, and further satisfying the following conditions: ##EQU4##  where, Σ d  denotes the distance between the frontmost surface and the rearmost surface of said rear conversion lens; f R  represents the focal length of said rear conversion lens; B f  is the back focus when said rear conversion lens is mounted on the objective lens; d 0  designates the distance between the frontmost surface of said rear conversion lens and the rear focal point of said objective lens; n --  denotes the refractive index of the negative lens which has the highest refractive index in said rear conversion lens; and ν --   represents the Abbe number of the negative lens which has the lowest Abbe number in the rear conversion lens.   
     
     
       2. The rear conversion lens as set forth in claim 1, wherein said positive lens in said front lens unit nearest to the object satisfies the following condition:   -1.7<q.sub.1 <-0.2                                         (6)     (where: q 1  represents a shape factor of the positive lens, which is defined as ##EQU5## r 1  being a radius of curvature of the lens surface to the side of the object, and r 2  being a radius of curvature of the lens surface to the side of the image).   
     
     
       3. The rear conversion lens at set forth in claim 2, wherein said lens satisfies the following conditions:   ______________________________________                                    
       n.sub.1 < 1.65,                                                    
                     ν.sub.1 < 40                                      
       n.sub.2 < 1.75,                                                    
                     ν.sub.2 > 40                                      
       n.sub.3 < 1.65,                                                    
                     ν.sub.3 < 40                                      
       n.sub.4 > 1.82,                                                    
                     ν.sub.4 > 35                                      
______________________________________                                    
     (where: n 1 , n 3  and ν 1 , ν 3  respectively denote the refracive indexes and the Abbe numbers of the two positive lenses in said front lens unit; n 2  and ν 2  represent respectively the refractive index and the Abbe number of the negative lens in said front lens unit, and n 4  and ν 4  respectively designate the refractive index and the Abbe number of the negative lens in the rear lens unit).   
     
     
       4. The rear conversion lens as set forth in claim 3, wherein said lens satisfies the following conditions:   ______________________________________                                    
β = 1.4142                                                           
              f.sub.R = -95.0774                                          
______________________________________                                    
          d.sub.0 = -36.60543                                             
r.sub.1 = ∞                                                         
          d.sub.1 = 2.7                                                   
                      n.sub.1 = 1.59507                                   
                                 ν.sub.1 = 35.5                        
                                         L.sub.1                          
r.sub.2 = -45.265                                                         
          d.sub.2 = 0.5                                                   
r.sub.3 = -120.630                                                        
          d.sub.3 = 1.0                                                   
                      n.sub.2 = 1.84042                                   
                                 ν.sub.2 = 43.3                        
                                         L.sub.2                          
r.sub.4 = 33.259                                                          
          d.sub.4 = 1.0                                                   
r.sub.5 = 29.167                                                          
          d.sub.5 = 4.2                                                   
                      n.sub.3 = 1.59507                                   
                                 ν.sub.3 = 35.5                        
                                         L.sub.3                          
r.sub.6 = -132.889                                                        
          d.sub.6 = 2.2                                                   
r.sub.7 = -31.846                                                         
          d.sub.7 = 1.3                                                   
                      n.sub.4 = 1.90265                                   
                                 ν.sub.4 = 35.8                        
                                         L.sub.4                          
r.sub.8 = -80.547                                                         
          Bf = 38.8895                                                    
 Σ .sub.d = 12.9                                                    
           ##STR9##                                                       
 ##STR10##                                                                
           q.sub.1 = -1.0                                                 
______________________________________                                    
     (where: β denotes magnification of said rear conversion lens; f R  represents focal length of said rear coversion lens; d 0  indicates a distance between the frontmost surface of the rear conversion lens and the focal point at the rear side of said objective lens; r 1  to r 8  are respectively the radius of curvature of the lens components; d 1  to d 7  are respectively the central thickness and air spaces of the successive lens components; n 1  to n 4  the refractive index of the lens components; ν 1  to ν 4  the Abbe number of the components lenses; L 1  a positive lens with its surface having a greater curvature faced to the image side, L 2  is a negative biconcave lens, and L 3  is a positive lens with its surface having a greater curvature facing to the object side, all these three lenses constituting the front lens unit; and L 4  is a negative lens constituting the rear lens unit; B f  is a back focus when said rear conversion lens is mounted on the objective lens; Σ d  denotes a distance between the frontmost surface and the rearmost surface of said rear conversion lens; and q 1  represents a shape factor of the positive lens, which is defined as ##EQU6## r s  being a radius of curvature of the lens surface to the side of the object, and r t  being a radius of curvature of the lens surface to the side of the image).   
     
     
       5. The rear conversion lens as set forth in claim 3, wherein said rear lens unit further includes a positive lens to the image side of said negative lens. 
     
     
       6. The rear conversion lens as set forth in claim 5, wherein said positive lens in said rear lens unit satisfies the following condition:   n.sub.5 <1.56, ν.sub.5 >44     (where: n 5  is refractive index, and ν 5  is Abbe number of said positive lens).   
     
     
       7. The rear conversion lens as set forth in claim 6, wherein said lens satisfies the following conditions:   ______________________________________                                    
β = 1.4 f.sub.R = -104.8292                                          
______________________________________                                    
          d.sub.0 = -37.64194                                             
r.sub.1 = 110.088                                                         
          d.sub.1 = 3.4                                                   
                      n.sub.1 = 1.59507                                   
                                 ν.sub.1 = 35.5                        
                                         L.sub.1                          
r.sub.2 = -37.390                                                         
          d.sub.2 = 1.0                                                   
r.sub.3 = -40.473                                                         
          d.sub.3 = 1.0                                                   
                      n.sub.2 = 1.78797                                   
                                 ν.sub.2 = 47.5                        
                                         L.sub.2                          
r.sub.4 = 31.842                                                          
          d.sub.4 = 1.0                                                   
r.sub.5 = 35.163                                                          
          d.sub.5 = 3.8                                                   
                      n.sub.3 = 1.59507                                   
                                 ν.sub.3 = 35.5                        
                                         L.sub.3                          
r.sub.6 = -132.889                                                        
          d.sub.6 = 1.0                                                   
r.sub.7 = -52.00                                                          
          d.sub.7 = 1.0                                                   
                      n.sub.4 = 1.90265                                   
                                 ν.sub.4 = 35.8                        
                                         L.sub.4                          
r.sub.8 = 611.943                                                         
          d.sub.8 = 0.2                                                   
r.sub.9 = 50.800                                                          
          d.sub. 9 = 3.0                                                  
                      n.sub.5 = 1.49782                                   
                                 ν.sub.5 = 82.3                        
                                         L.sub.5                          
r.sub.10 = 2442.404                                                       
          Bf = 38.6208                                                    
 Σ.sub.d = 15.4                                                     
           ##STR11##                                                      
 ##STR12##                                                                
           q.sub.1 = -0.493                                               
______________________________________                                    
     (where: β denotes magnification of said rear conversion lens; f R  represents focal length of said rear conversion lens; d 0  indicates a distance between the frontmost surface of the rear conversion and the focal point at the rear side of said objective lens; r 1  to r 10  are respectively the radius of curvature of the lens components; d 1  to d 9  are respectively the central thickness of the lens components; n 1  to n 5  the refractive index of the lens components; ν 1  to ν 5  the Abbe number of the components lenses; L 1  a positive lens with its surface having a greater curvature faced to the side of the image, L 2  is a negative biconcave lens, and L 3  is a positive lens with its surface having a greater curvature faced to the side of the object, all these three lenses constituting the front lens unit; L 4  is a negative lens, and L 5  is a positive lens, these lenses constituting the rear lens unit; B f  is a back focus when said rear conversion lens is mounted on the objective lens; Σ d  denotes a distance between the frontmost surface and the rearmost surface of said rear conversion lens; and q 1  represents a shape factor of the positive lens, which is defined as ##EQU7## r s  being a radius of curvature of the lens surface to the side of the object, and r t  being a radius of curvature of the lens surface to the side of the image).   
     
     
       8. The rear conversion lens as set forth in claim 6, wherein said lens satisfies the following conditions:   ______________________________________                                    
β = 1.4 f.sub.R = -111.8457                                          
______________________________________                                    
          d.sub.0 = -37.63874                                             
r.sub.1 = 130.108                                                         
          d.sub.1 = 3.4                                                   
                      n.sub.1 = 1.59507                                   
                                 ν.sub.1 = 35.5                        
                                         L.sub.1                          
r.sub.2 = -35.344                                                         
          d.sub.2 = 1.0                                                   
r.sub.3 = -38.314                                                         
          d.sub.3 = 1.0                                                   
                      n.sub.2 = 1.78797                                   
                                 ν.sub.2 = 47.5                        
                                         L.sub.2                          
r.sub.4 = 28.774                                                          
          d.sub.4 = 1.0                                                   
r.sub.5 = 31.038                                                          
          d.sub.5 = 3.8                                                   
                      n.sub.3 = 1.59507                                   
                                 ν.sub.3 = 35.5                        
                                         L.sub.3                          
r.sub.6 = -132.855                                                        
          d.sub.6 = 1.0                                                   
r.sub.7 = -50.864                                                         
          d.sub.7 = 1.0                                                   
                      n.sub.4 = 1.90265                                   
                                 ν.sub.4 = 35.8                        
                                         L.sub.4                          
r.sub.8 = 535.743                                                         
          d.sub.8 = 0.2                                                   
r.sub.9 = 53.098                                                          
          d.sub. 9 = 3.0                                                  
                      n.sub.5 = 1.49782                                   
                                 ν.sub.5 = 82.3                        
                                         L.sub.5                          
r.sub.10 = -192.027                                                       
          Bf =  39.41941                                                  
 Σ.sub.d = 15.4                                                     
           ##STR13##                                                      
 ##STR14##                                                                
           q.sub.1 = -0.573                                               
______________________________________                                    
     (where: β denotes magnification of said rear conversion lens; f R  represents focal length of said rear conversion lens; d 0  indicates a distance between the frontmost surface of the rear conversion lens and the focal point at the rear side of said objective lens; r 1  to r 10  are respectively the radius of curvature of the lens components; d 1  to d 9  are respectively the central thickness of the lens components; n 1  to n 5  the refractive index of the lens components; ν 1  to ν 5  the Abbe number of the component lenses; L 1  a positive lens with its surface having a greater curvature faced to the side of the image, L 2  is a negative biconcave lens, and L 3  is a positive lens with its surface having a greater curvature faced to the side of the object, all these three lenses constituting the front lens unit; L 4  is a negative lens, and L 5  is a positive lens, these lenses constituting the rear lens unit; B f  is a back focus when said rear conversion lens is mounted on the objective lens; Σ d  denotes a distance between the frontmost surface and the rearmost surface of said rear conversion lens; and q 1  represents a shape factor of the positive lens, which is defined as ##EQU8## r 5  being a radius of curvature of the lens surface to the side of the object, and r t  being a radius of curvature of the lens surface to the side of the image).   
     
     
       9. The rear conversion lens as set forth in claim 6, wherein said lens satisfies the following conditions;   ______________________________________                                    
β = 1.4324                                                           
              f.sub.R = -92.2451                                          
______________________________________                                    
          d.sub.0 = -36.40544                                             
r.sub.1 = 105.216                                                         
          d.sub.1 = 2.7                                                   
                      n.sub.1 = 1.59507                                   
                                 ν.sub.1 = 35.5                        
                                         L.sub.1                          
r.sub.2 = 52.031                                                          
          d.sub.2 = 1.0                                                   
r.sub.3 = -83.298                                                         
          d.sub.3 = 1.0                                                   
                      n.sub.2 = 1.78797                                   
                                 ν.sub.2 = 47.5                        
                                         L.sub.2                          
r.sub.4 = 29.263                                                          
          d.sub.4 = 1.0                                                   
r.sub.5 = 32.766                                                          
          d.sub.5 = 3.3                                                   
                      n.sub.3 = 1.59507                                   
                                 ν.sub.3 = 35.5                        
                                         L.sub.3                          
r.sub.6 = -195.060                                                        
          d.sub.6 = 1.5                                                   
r.sub.7 = -40.120                                                         
          d.sub.7 = 1.0                                                   
                      n.sub.4 = 1.90265                                   
                                 ν.sub.4 = 35.8                        
                                         L.sub.4                          
r.sub.8 = 414.484                                                         
          d.sub.8 = 0.2                                                   
r.sub.9 = 59.129                                                          
          d.sub.9 = 3.0                                                   
                      n.sub.5 = 1.53172                                   
                                 ν.sub.5 = 49.1                        
                                         L.sub.5                          
r.sub.10 = -292.539                                                       
          Bf = 37.6941                                                    
 Σ.sub.d = 14.7                                                     
           ##STR15##                                                      
 ##STR16##                                                                
           q.sub.1 = -0.338                                               
______________________________________                                    
     (where: β denotes magnification of said rear conversion lens; f R  represents focal length of said rear conversion lens; d 0  indicates a distance between the frontmost surface of the rear coversion lens and the focal point at the rear side of said objective lens; r 1  to r 10  are respectively the radius of curvature of the lens components; d 1  to d 9  are respectively the central thickness of the lens components; n 1  to n 5  the refractive index of the lens components; ν 1  to ν 5  the Abbe number of the component lenses; L 1  a positive lens with its surface having a greater curvature faced to the side of the image, L 2  is a negative biconcave lens, and L 3  is a positive lens with its surface having a greater curvature faced to the side of the object, all these three lenses constituting the front lens unit; and L 4  is a negative lens, and L 5  is a positive lens, these lenses constituting the rear lens unit; B f  is a back focus when said rear conversion lens is mounted on the objective lens; Σ d  denotes a distance between the frontmost surface and the rearmost surface of said rear conversion lens; and q 1  represents a shape factor of the positive lens, which is defined as ##EQU9## r s  being a radius of curvature of the lens surface to the side of the object, and r t  being a radius of curvature of the lens surface to the side of the image).   
     
     
       10. A compact rear conversion lens for a single lens reflex camera, said lens being removably disposed between an objective lens and a camera body to provide a composite focal length longer than the focal length of said objective lens, said compact rear conversion lens being capable of maintaining excellent image forming performance, even if it is mounted on an objective lens having a relatively short back focus, a large aperture ratio, and an exit pupil which is at a relatively distant position from the image plane, consisting of, in order from the object side: a first positive lens, a negative biconcave lens, a second positive lens, a negative meniscus lens convex to the image side, and further satisfying the following conditions: ##EQU10##  (where: Σ d  denotes the distance between the frontmost surface and the rearmost surface of said rear conversion lens; f R  represents the focal length of said rear conversion lens; B f  is the back focus when said rear conversion lens is mounted on the objective lens; and d 0  designates the distance between the frontmost surface of said rear conversion lens and the rear focal point of said objective lens).   
     
     
       11. A compact rear conversion lens according to claim 10, wherein said first positive lens has a surface of sharper curvature facing the image side, and said second positive lens has a surface of sharper curvature facing the object side.